This invention relates to electroplating technology, such as wafer electroplating. Even more specifically, the invention pertains to methods and apparatus for controlled-angle wafer handling for the purpose of improving electroplated wafer uniformity, quality, and throughput.
Electroplating has many applications. One very important developing application is in plating copper onto semiconductor wafers to form conductive copper lines for xe2x80x9cwiringxe2x80x9d individual devices of the integrated circuit. Often this electroplating process serves as a step in the damascene fabrication procedure.
A continuing issue in modern VLSI wafer electroplate processing is quality of the deposited metal film. Given that metal line widths reach into the deep sub-micron range and given that the damascene trenches often have very high aspect ratios, electroplated films must be exceedingly homogeneous (chemically and physically). They must have uniform thickness over the face of a wafer and must have consistent quality across numerous batches.
Some wafer processing apparatuses are designed to provide the necessary uniformity. One example is the clamshell apparatus available in the SABRE(trademark) electroplating tool from Novellus Systems, Inc. of San Jose, Calif. and described in U.S. Pat. Nos. 6,156,167, 6,159,354 and 6,139,712, which are herein incorporated by reference in their entirety. The clamshell apparatus provides many advantages in addition to high wafer throughput and uniformity; such as wafer back-side protection from contamination during electroplating, wafer rotation during the electroplating process, and a relatively small footprint for wafer delivery to the electroplating bath (vertical immersion path).
There are many factors that can effect the quality of an electroplating process. Of particular note in the context of the present invention are problems having their genesis in the process of immersing the wafer into an electroplating bath. As indicated, bubbles can be entrapped on the plating underside of the wafer (the active side) upon immersion. This is especially true when the wafer is immersed in a horizontal orientation (parallel to a plane defined by the surface of the electrolyte) along a vertical immersion trajectory. Depicted in FIG. 1A is a cross-sectional diagram of a typical bubble-entrapment scenario arising in an electroplating system 101. A horizontally oriented wafer 103 is lowered towards an electrolyte 107 in a vessel 105 along a vertical Z-axis and ultimately immersed in the electrolyte. Vertical immersion of horizontally oriented wafer 103 results in air bubbles 109 being trapped on the underside (plating surface) of wafer 103.
Air bubbles trapped on the plating surface of a wafer can cause many problems. Bubbles shield a region of the plating surface of a wafer from exposure to electrolyte, and thus produce a region where plating does not occur. The resulting plating defect can manifest itself as a region of no plating or of reduced thickness, depending on the time at which the bubble became entrapped on the wafer and the length of time that it stayed entrapped there. In an inverted (face down) configuration, buoyancy forces tend to pull bubbles upwards and onto the wafer""s active surface. They are difficult to remove from the wafer surface because the plating cell has no intrinsic mechanism for driving the bubbles around the wafer edges, the only path off the wafer surface. Typically, wafer 103 is rotated about an axis that passes through its center and is perpendicular to its plating surface. This also helps to dislodge bubbles through centrifugal force, but many of the smaller bubbles are tenacious in their attachment to the wafer.
Therefore, while horizontal wafer orientation (especially coupled with a vertical immersion trajectory) has numerous advantages from a hardware configuration and throughput standpoint, it leads to technically challenging issues associated with gas entrapment and consequent defect formation.
One way to facilitate removal of entrapped bubbles is to use a vertically directed electrolyte flow aimed at the plating surface of the wafer. This can help dislodge the bubbles. As depicted in FIG. 1B, scenario 102, plating solution is directed from a conduit 111 normal to the plating surface of the wafer at a velocity sufficient to dislodge entrapped bubbles. As indicated by the arrows emanating from 111, the majority of the flow is directed at the center of wafer 103. As the flow encounters the surface of the wafer, it is deflected across the wafer surface to push the bubbles toward the sides of wafer 103 as indicated by the dashed arrows. This helps remove bubbles that are not only generated upon immersion, but also those formed or reaching the surface during electroplating. Unfortunately, the radial non-uniformity of the forced convection of such systems can result in non-uniform plating profiles. This is because the electroplating rate is a function of local fluid velocity, and the forced convection of the systems such as depicted in FIG. 2B introduces non-uniform velocity profiles across the wafer surface.
Another problem associated with vertical immersion of a horizontally oriented wafer is multiple wetting fronts. When a wafer is immersed in this way, the electrolyte contacts the wafer at more than one point, creating multiple wetting fronts as the wafer is submerged in the electrolyte. Where individual wetting fronts converge, bubbles may be trapped. Also, defects in the finished plating layer can be propagated from microscopic unwetted regions formed along convergence lines of multiple wetting fronts.
What is needed therefore is a way to improve plated metal quality. Improved methods and apparatus should reduce the problems that can arise from bubble formation and multiple wetting fronts during wafer immersion.
The present invention provides methods and apparatus for controlling the orientation of a wafer with respect to the surface of an electrolyte during an electroplating process. A wafer is delivered to an electrolyte bath along a trajectory normal to the surface of the electrolyte. Along this trajectory, the wafer is angled before entry into the electrolyte for angled immersion. A wafer can be plated in an angled orientation or not, depending on what is optimal for a given situation. Also, in some designs, the wafer""s orientation can be adjusted actively during immersion or during electroplating. Active angle adjustment refers to changing the angle of the wafer at any time during positioning or plating. This provides flexibility in various plating scenarios.
Stated somewhat differently, the invention provides methods and apparatus for wafer movement that embody two movements: first, moving the wafer into and out of a plating bath along a trajectory substantially normal to the surface of the electrolyte; and second, adjusting the angle of the wafer with respect to the surface of the electrolyte. These discrete movement functions are performed either concurrently or separately. Apparatus for performing the two movements have two actuators, one for each movement; therefore the movements can be separately controlled depending on the electroplating process demands. In one example, a wafer is tilted as it is immersed in a plating bath; in another example the wafer is tilted and then directed into the plating bath. In yet another example, the wafer is tilted to a new angular orientation after it is immersed in the bath at a first angular orientation.
One aspect of this invention pertains to methods of positioning a wafer during electroplating. In one preferred embodiment, a wafer is loaded and/or unloaded from an electroplating apparatus in a horizontal orientation relative to the surface of the plating bath. During electroplating, the angle of the wafer is actively changed to orientations that are optimal for reasons particular to each electroplating event. The wafer is rotated or not, depending on the desired characteristics of the plated metal film.
Of particular importance to the quality of the deposited metal film on a wafer with respect to the methods of the invention are four factors: (1) the linear speed at which the wafer is immersed and withdrawn from an electroplating bath, (2) the rotational speed of the wafer, (3) the angle of the wafer surface with respect to the electrolyte surface, and (4) the xe2x80x9cswing speedxe2x80x9d or angular speed upon tilting of the wafer. All of these factors (and combinations thereof) have important ramifications to the quality of the deposited metal film resulting from methods herein, as will be discussed below.
Methods of the invention utilize wafer immersion into an electrolyte along a vertical trajectory; that is, along an axis substantially normal to the electrolyte surface. Of particular importance is the speed at which this immersion (or extraction) of the wafer is performed. If the speed is too fast, then the plating process and ultimately the deposited film quality will suffer. If the speed is too slow, then throughput suffers. Preferably the speed for immersion and extraction of a wafer is between about 5 and 50 millimeters/second.
Rotation speed of a wafer is important for a number of reasons. Of particular importance is the speed of rotation during immersion and plating. If the wafer is rotated too quickly during immersion, frothing of the electrolyte can form bubbles that become entrapped on the wafer surface causing defects in the plated metal film. If the wafer is rotated too slowly, then film homogeneity may suffer. Preferably the rotational speed for immersion and extraction of a wafer is between about 50 and 150 rpm, depending on the wafer diameter. Larger diameter wafers are generally rotated more slowly than smaller diameter wafers.
The tilt angle of the wafer is important as a means to allow bubbles to escape that otherwise would become entrapped on the wafer surface. Angled immersion is important also with respect to wetting fronts formed upon immersion of the wafer into a plating bath. The preferred angle for this invention has been found to be about 5 degrees or less from horizontal.
The swing speed or angular speed upon tilting of the wafer is important for example when a wafer is already immersed in a plating solution and the tilt angle is changed. If the angle is changed too quickly, the plating solution may be splashed or agitated to a state of frothing. Again, this causes bubbles which are detrimental to the plating process and are to be avoided. Preferably, the swing speed is between about 0.25 and 3 degrees per second.
As mentioned, the invention finds particular use in the context of copper electroplating. In modem damascene processing, conditions have to be increasingly stringent for optimal plating quality and throughput. The invention provides plating environments with less possibility for defects caused by bubbles and multiple wetting fronts.
Another aspect of this invention pertains to apparatus for implementing the method of the invention. Apparatuses of the invention include one component that can tilt the wafer with respect to the surface of the electrolyte during vertical positioning in the electroplating process and another component that can translate the wafer vertically into and out of the electrolyte. This allows a wafer holder component to be tilted as well as lifted in and out of the electrolyte bath, using separate actuators for each movement component. In a preferred embodiment, one component of the apparatus has an xe2x80x9cinverted pendulumxe2x80x9d configuration that allows such tilting. In this embodiment, a wafer holder component is tilted by moving its distal end (the end away from the wafer) along an arced track with movement provided by a first actuator. The proximal end of the wafer holder provides a fixed pivot point at or near the wafer. The wafer holder, arced track, and first actuator form an assembly that is moved along a vertical trajectory driven by a second actuator, which provides substantially linear bi-directional movement of the wafer into or out of an electrolyte bath.
These and other features and advantages of the present invention will be described in more detail below with reference to the associated figures.